There are several widely used laser spectroscopy techniques which involve measurement of absorption of the laser light within a sample. In order to achieve high sensitivity in many cases the measurement signals have to be extracted from noisy background. This usually involves phase sensitive detection of the modulated laser radiation. PAS (photoacoustic spectroscopy) is an analytical method that involves stimulating a sample with modulated light and detecting the resulting sound waves emanating from the sample. A photoacoustic measurement can be made as follows. First, light is used to excite molecules within a sample. Such excitation can include, for example, absorption of the light by the molecule to change an energy state of the molecule. As a result, the energized molecule enters an excited state. Optical excitation is followed by energy transfer processes (relaxation) from the initially excited molecular energy level to other degrees of freedom, in particular translational motion of the fluid molecules. During such relaxation, heat, light, volume changes and other forms of energy can dissipate into the environment surrounding the molecule. Such forms of energy cause expansion or contraction of materials within the environment. As the materials expand or contract, sound waves are generated.
In order to produce sound waves, or photoacoustic signals, the light is modulated at a specific acoustically resonant modulation frequency f (having a modulation period 1/f), sometimes also referred to herein as ω. The sample environment can be enclosed and may be constructed to resonate at the modulation frequency. An acoustic detector mounted in acoustic communication with the sample environment can detect changes occurring as a result of the modulated light excitation of the sample. Because the amount of environmental change associated with the absorbed energy is proportional to the concentration of the absorbing molecules, the photoacoustic signal can be used for concentration measurements.
In typical PAS, a resonant acoustic cavity or sample cell is used to isolate and amplify sound wave signals, thereby increasing sensitivity of detection. The light intensity or wavelength is modulated at a frequency, f. The absorbed energy is accumulated in the acoustic mode of the sample cell during oscillation periods. Quartz enhanced photoacoustic laser spectroscopy (QEPAS) has been found to be highly sensitive and selective technique for the detection of gas concentrations at the parts-per-billion (ppb) and parts-per-trillion (ppt) level. Because of its sensitivity, QEPAS may be useful in many different applications. A number of applications, however, require an ultra-compact footprint i.e. small size, and low power consumption.
Currently, systems are based on modular architectures with an externally mounted laser source, separate power and thermal controllers, environmental transducers, and/or separate processing hardware and software. Such systems require human feedback to operate and may not be considered to be truly integrated. In addition, at present, sensors typically utilize separate sub-system controllers running independently. Systems with independent sub-systems, as such, cannot be considered fully integrated. Because each sub-system requires a respective controller, present systems are bulky, expensive, and require impractical amounts of electrical power.
Consequently, there is a need for a fully integrated trace-gas sensor platform which is low cost, compact, and power efficient.